1. Field of the Invention
This invention relates generally to electronic article surveillance (EAS) systems and, more particularly, to a system and method for synchronizing transmissions in an EAS system.
2. Description of the Related Art
In acoustomagnetic or magnetomechanical electronic article surveillance (EAS) systems, both detection and deactivation units may be provided. Both units typically excite an EAS tag by transmitting an electromagnetic burst at a resonance frequency of the tag. When the tag is present within the electromagnetic field created by the transmission burst, and has not been deactivated, the tag begins to resonate with an acoustomagnetic or magnetomechanical response frequency that is detectable by a receiver in both detection units. The detection unit may then provide some type of signal, for example, an alarm signal indicating the detection of a response from an EAS tag. The deactivation units also typically transmit a deactivation signal to deactivate the EAS tag such that the EAS tag will not resonate with an acoustomagnetic, magnetomechanical or electromagnetic response frequency when the EAS tag is present in the electromagnetic field of the detection units.
In EAS systems, the transmitter burst signal typically does not end abruptly, but instead decays exponentially because of transmitter circuit resonance. If the transmissions from nearby units are not synchronized, false detections may occur because all units transmit and receive at the same frequency. These false detections can result in false alarms and/or false deactivations.
In order to synchronize the transmissions from the detection and deactivation units of the EAS system, a manual synchronization process is typically performed. Specifically, field service personnel use, in connection with a configuration program, phasing tools that include two loop antennae and an oscilloscope to synchronize each of the units. The synchronization is provided by changing a delay time for the unit to transmit referenced from the AC zero-crossing point of the unit. This procedure is repeated for every deactivation and detection unit, for example, in a retail store.
However, because the wiring of the AC power supply to each of the units may be different, for example, the phase may be shifted by 120 degrees depending on how the power supply is wired (e.g., how the power outlet is wired), the AC zero-crossings can be different. This can result in improper synchronization of the units because the zero-crossings are out of phase. Further, isolation transformers for each unit can also affect the required delay for synchronization with the other units. Thus, the manual synchronization process is not only time consuming and susceptible to human error, for example, in reading the oscilloscope, but the reference for synchronizing the units may be different because of wiring differences in the power supply. Out of phase synchronization can thereby result.
Other known systems or processes for synchronizing the units within the EAS system provide for communicating the exact time of transmission for each of the units or use a reference signal transmitted by a broadcast transmitter to synchronize the units. However, because of internal delays within the units and other transmission delays, these processes often fail to adequately synchronize the units and are costly to implement. Further, a reliable twenty-four hour per day reception of signals from a selected FM or TV broadcast station is needed for providing a reference signal from a broadcast transmitter. This adds complexity and cost to the system.
Units within an EAS system also may be synchronized by periodically shutting down transmissions and then listening for other EAS transmitters from which a delay between the received signals and a AC zero-crossing of the shut down unit is determined. However, this process may not satisfactorily synchronize deactivator and detector units because of the large difference in transmit power and antenna size for these different types of units.